The present invention relates to a superconductized semiconductor device with use of proximity effect superconductivity, and more particularly concerns a superconductized semiconductor device preferable for use as a Josephson junction device in a semiconductor memory circuit or a logic circuit which operates at an ultra high speed with small power consumption in an electronic computer, for use as a light emitting device which is represented by a high speed modulation semiconductor laser device used in an optical fiber communication system, an optical information processing system, or the like, or for use as a light emitting device such as a photodiode having a high speed response characteristic.
First, the following describes electronic devices. Since a Josephson effect was predicted by B. D. Josephson in Physics Letters, Vol. 1, 1962, page 251, the Josephson junction device has been developed as a memory IC or logic IC of ultra high speed and ultra low power consumption. For details, as an example, see "Ultra High Josephson Devices", edited by Yoshio Hayakawa, Baifukan, 1986. The existing Josephson junction ICs face the following problems.
1. It is very difficult to make a film thickness of an ultra thin insulating layer of 1 to 3 nm with high quality and uniformity in the superconductor-insulator-superconductor (S-I-S) structure. In fact, in the Josephson junction, the tunnel Josephson current, Io, is widely dispersed as reflected by dispersion of the film thickness. It is also difficult to form LSIs of high yield as short circuits happen between the superconductors. This is due to pin holes appearing in the insulation layer and the like.
2. It is essentially difficult to make the tunnel Josephson current, Io, higher as the potential wall of the real insulation layer is as high as several eV. The result is that it is impossible to have a large current density with fine Josephson junction. The operation speed is made to slow down in a highly integrated LSI. The advantage of the high integration cannot be utilized.
However, although a Josephson junction IC has such problems, it provides a great merit which is not in the semiconductor device. It is very advantageous in that it can be made compact with very high integration. This is due to ultra low power consumption. This advantage leads to a possibility that the system such as an ultra super high speed super computer can be built in a compact structure.
An attempt was made to overcome above described problem 1 of the Josephson junction device, for example, in the Japanese Patent Laid-Open 57-176780. In this disclosure, the Josephson junction device is formed using a pn junction of degenerate semiconductors, for example, SnTe, GeTe, PbTe, and SrTiO.sub.2, of which materials show superconductivity.
However, those superconducting degenerate semiconductors are defective in that they are limited in use of materials. The superconducting critical temperature is as low as 100 m.degree.K. and the p type and n type carrier concentrations cannot be well controlled even for the superconducting degenerate semiconductors (the known ones of which are 1 limited in SnTe, GeTe, PbTe, SrTiO.sub.2, and the like). As a result, the pn junction depletion layer thickness as the insulation layer is dispersed. This results in that the superconducting current is dispersed. Particularly, the usual silicon and GaAs degenerate semiconductors now used as electronic device LSIs, as is generally known, cannot be made to become either of the n type or p type superconducting state even if they are cooled to 10 .mu.K which is the lowest temperature we can reach.
Also, a field effect transistor with use of superconduction and semiconduction proximity effect was proposed by T. D. Clark et al., "Feasibility of Hybrid Josephson Field Transistor", Journal of Applied Physics, Vol. 51, No. 5, pp2736-2743, May 1980. After that, the idea of T. D. Clark et al. was affirmatively verified in the experiment with use of Si by Nishino et al., Physical Review, Vol. B33, page 2042, 1986. The MOS-JOFET of T. D. Clark was an epoch making proposal in the sense that the semiconductor device was combined with the superconducting device.
However, there are two essential problems in applying the combined device to LSI. One is that the flowing superconducting current is too small to drive the wire capacitance and transistor capacitance when it is used in a practical circuit. In other words, its operation speed is slow, or it has no realistic merit. The other is that the superconducting current can be observed only in very minute gate length of around 0.1 .mu.m. This means that a realistic circuit cannot be fabricated unless an ultra microprocessing technique can be fully used. This is not preferable for the silicon LSI which is to be increased in the integration in generation by generation. The problem that the sufficient superconducting current cannot be obtained is an essential defect based on the operational principle of the device.
Further, it was proposed in one disclosure that the existing S-I-S Josephson device was formed on silicon substrate to combine with the usual silicon device (for example, see the Japanese Patent Laid-Open 64-86576).
However, that disclosure does not resolve the above-described problems as the Josephson junction device itself is left in the existing structure. It is a feature that demand for reliability of an individual device, as seen in Si and GaAs, is strict, particularly in LSI. It is the most important for LSI to fully utilize its performance rather than the characteristics of the individual device.
On the other hand, as is well known, the silicon LSIs (large scale integrated circuits) include CMOS (complimentary metal oxide semiconductor) of large scale and low power consumption, bipolar ECL (emitter coupled logic) LSI of high speed and high power consumption, and Bi-CMOS LSI which is intermediate between the both LSIs.
However, the silicon LSIs have not achieved ultra high speed, large scale, and low power consumption.
Furthermore, attention is recently attracted to LSIs of FETs using compound semiconductor such as GaAs, including the so-called HEMT (high electron mobility transistor) LSI, and an HBT (heterojunction bipolar transistor). The FET is defective in that a little load driving capacity prevents actual high integration. The HBT also is defective in that high heat generation prevents actual high integration. It is not possible, that is, to achieve an LSI of ultra high speed, large scale, and low power consumption with the compound semiconductor FET or bipolar by itself.
Relationship between the power consumption and speed of the electronic devices used up to date is shown in FIG. 9, which is taken from Physical Society of Japan, "Supercomputers", p228, Baifukan, 1985.
In turn, this and the following paragraphs describe optical devices. Usual light emitting diodes can operate in the way such that a forward current is made to flow in the pn junction of the semiconductor, holes are injected into the valence band of the semiconductor, electrons are injected in the conduction band, and they are recombined directly or through the center of light emission to emit the light. The energy of the light emitted from the semiconductor light emitting diode is equal to or lower than the energy corresponding to the band gap, Eg, of the semiconductor material in the active layer region.
The situation is same as in the semiconductor laser. In the semiconductor laser, forward current is made to flow in the pn junction of a direct transition type semiconductor, holes are injected into the valence band of the semiconductor, electrons are injected into the conduction band, a population inversion is formed, the holes and electrons are recombined to emit light, and the recombined emission light is amplified as stimulated emission in a resonant cavity to oscillate laser light. The energy of the light emitted from the semiconductor laser, therefore, is limited to around the band gap, Eg, of the semiconductor material in the active layer region.
In the laser using the usual III-V semiconductor material, wavelengths can be selected to some degree by selecting semiconductor materials and by controlling the band gap with crystal mixturization of them. The energy of the light emitted, however, was around 0.3 eV to 2.5 eV.
The following lists some problems of the existing light emitting and receiving devices. The first problem is associated with the laser light of high energy. To achieve higher energy of laser light, the following two attempts have been made.
One is use of semiconductor materials having greater band gaps. The materials available include II-VI semiconductors, such as CdTe, ZnSe, ZnS, and IV semiconductors, such as SiC and diamond. Parts of these materials cannot be practically employed as they need a high voltage power supply for generation of electron ray and a vacuum atmosphere, although laser oscillation was confirmed in the electron beam enhancement. We have not developed yet a process for obtaining a quality crystal which can be available for use as semiconductor light emitting diode or semiconductor laser. We also have not established yet a method of doping. Though there are examples of the light emitting diode with use of the materials mentioned above, they have a problem that their light emission efficiencies are too low. (For details, for example, see Shigeo Fujita, Oyobutsuri, Vol. 54, pp40-47, 1985.)
In other attempts a method provides that a light of semiconductor laser is irradiated onto a nonlinear optical crystal to emit higher harmonics by way of the nonlinear optical effect. This method also is defective in that the apparatus cannot be made smaller as a multiple of components of the semiconductor laser and nonlinear optical crystal are used. It further is defective in that it cannot obtain a high output power as the emission efficiency of the higher harmonics is too low. (For details, see Takatomo Sasaki, Oyobutsuri, Vol. 58, pp895-903, 1989.)
The second problem in the usual light emitting and receiving devices is that a threshold current of the laser oscillation is to be decreased. In order to efficiently start the laser oscillation at a low threshold current, in general, it is necessary to make larger the shape of the carrier energy distribution in the vicinity of the band gap. In the existing semiconductor laser, for example, the active layer makes use of a quantum well structure in place of the semiconductor bulk crystal to achieve lower threshold current and more narrow spectra. First, the following describes principles of the operation. Let us consider the distribution states of electrons and holes with the same amount of electrons enhanced in the two cases: the bulk semiconductor crystal and quantum well structure semiconductor. FIGS. 2A and 2B show dependency of the density of states (axis of abscissas) on the energy (axis of ordinates)of the bulk semiconductor and quantum well structure semiconductor respectively. The curve 201 indicates a density of states of the conduction band, 202 is a density of states of the valence band, 203 is an energy distribution of the electrons, and 204 is an energy distribution of the holes. Owing to change of the bulk to quantum well structure, the energy distributions of electrons and holes are changed. The quantum well structure has larger energy distribution around the band gap than the bulk. This can explain that the oscillation threshold value of the semiconductor laser using the quantum well structure is lower. (For details and examples, see Okamoto et al., Oyobutsuri, Vol. 52, pp843-851, 1983.)
Another known method for changing the dependency of the density of states on the energy is to use of quantization of cyclotron movement of carriers in a strong magnetic field. (For details, see the Japanese Patent Laid-Open 64-68994 and Okamoto et al., Oyobutsuri, Vol. 52, pp-843-851, 1983.) The density of states of the bulk semiconductor shown in FIG. 2A is split into Landau levels as shown in FIG. 2C. In this case, also, the density of states rises up sharply at the band edges. It is clear that the effect obtained is similar to that of the quantum well structure. However, it is not realistic in view of prevention from small size and adverse effect of magnetic field to other units in that it has to coexist with a magnetic field generator, including a superconducting magnet.
In the above examples where the density of states is changed by the quantum well structure or magnetic field, the dependency of the density of states on the energy can be made sharp at the band edge. It however cannot make sharp the carrier density distribution on the high energy side that is determined by the injected carrier concentration.
As an example which is fundamentally different in nature from the ones mentioned above, a laser using light emission from a Bose condensation system of exiton was proposed. (See the Japanese Patent Laid-Open 61-218192.) The following describes a principle of the proposed laser briefly. Electrons and holes are two-dimensionally accumulated on both sides of a tunnel wall layer formed in a heterojunction semiconductor, respectively. FIG. 2D shows a band diagram and carrier distribution state in that condition. The electron 216 and hole 217 separated by a tunnel barrier wall 215 in a semiconductor 213 and 214 of narrower band gap enclosed by a semiconductor 211 and 212 of wider band gap form an exiton by an Coulomb interaction. The electron and hole can be made to recombine to emit light as tunneling the barrier wall when the film thickness of the tunnel barrier layer 215 is optimized. The emitted light is induction amplified by a resonant cavity to make the laser oscillate.
If the exiton system is Bose condensed at a low temperature, a superradiance laser can be accomplished. The superradiance laser provides 1/10 of light spectrum width and 1/10000 of laser oscillation threshold current as compared with the usual semiconductor laser. In the superradiance laser, however, the electron and hole to be recombined are separated spatially, and the recombination probability is dominated by overlap of wave functions in the tunnel barrier layer. Its light intensity, therefore, cannot be theoretically made high. As it also is essentially difficult to make the concentration of the exiton system high, the Bose condensation cannot be made at a practical temperature, which is higher than 10.degree. K. or so.
Similarly, a laser using a superconductor is known as a Bose condensed electron system. There are two operational principles in the type of laser. One is that an electron quasi-particle and hole quasi-particle are injected into the superconductor having a gap .DELTA., and when they are recombined a light of 2.DELTA. is radiated and laser oscillated. (For details, for example, see the Japanese Patent Laid-Open 63-302581 and the Japanese Patent Laid-Open 63-263783.) This concept is based on an idea that in the superconductor, the quasi-particle having an energy of 2.DELTA. higher than the ground state is population inverted, the energy being caused by decay of the Cooper pair.
However, it is theoretically difficult that the ground state and quasi-particle state are population inverted by injecting the electron and hole with the superconducting state kept. Even if it can made laser oscillation, the light radiated from the laser is limited in microwave to infrared ray as the existing known superconductor has a gap energy of several meV to 50 meV at most.
In the other operational principle of a laser, a superconductor and a transparent insulating film are laminated to form a Josephson junction. A potential .delta. V is applied across the superconductor partitioned by the insulating film to radiate a photon having an energy of 2q.delta. V, where q is a unit charge. (For details, for example, see the Japanese Patent Laid-Open 63-316493 and the Japanese Patent Laid-Open 63-262882.)
However, in the S-I-S structure, the wave functions of the Cooper pair cannot be overlapped in the same position of the insulator. In fact, it can obtain little light emission. It also is very difficult to make the insulating layer to have a high quality and uniformity with ultra thin single crystal film of 1 to 3 nm. It is dominated by a non-radiative transition through a trap in the insulating layer. The laser therefore has not be realized yet as light emitting device.
A third problem of the existing light emitting and receiving devices is that the relaxation frequency, fr, is too low. In the ultra high speed optical communication, a high relaxation frequency is indispensable. Even in the existing modulation doping-multiple quantum well laser which can operate at the highest speed, it is 40 GHz at most. (For details, for example, see Kazuhisa Uomi, Oyobutsuri, Vol. 57, pp708-713, 1988.) In the usual laser, it is around 10 GHz.
There is no example that light emission and reception processes from a coherent electronic system have been experimentally proved. There are many points to be elucidated as to the the basic process of light emission and reception.